Malformation of the outflow tract (OFT) is one of the most common congenital heart defects (CHD) in humans and a leading cause of childhood lethality. Their etiology, however, is largely unknown as most CHDs are thought to be complex genetic disorders arising from mutations in multiple genes. The long-term objectives of this project are to define how different pathways interact to regulate OFT morphogenesis, to identify the genetic mutations/ variants altering these pathways in humans, and to elucidate how the mutations/ variants interact to modify OFT malformations. The knowledge will guide us to design diagnostic and therapeutic approaches. In humans with 22q11.2 Deletion Syndrome (DS), heterozygous loss of TBX1, a T-box transcription factor, predisposes them to CHDs at a high rate of 65%. The highly variable clinical manifestation of OFT defects in 22q11.2DS individuals with the identical deletion has led to the hypothesis that genetic variants, residing outside the deletion and affecting TBX1-networks, may interact with TBX1 haploinsufficiency to modify the clinical presentation of OFT malformation. The objective of the current proposal is to understand how alteration of planar cell polarity (PCP) signaling, an evolutionarily conserved signaling mechanism that regulates tissue morphogenesis, may contribute to OFT malformation in 22q11.2DS patients. The objective will be achieved by specifically testing 1) how Tbx1 may regulate morphogenesis in cardiac progenitors in the second heart field (SHF) through PCP signaling to promote OFT formation in mice; 2) how 3 functionally validated PCP genes variants identified in 22q11.2DS patients may modify the OFT defects in Tbx1 mutant mice, and 3) how additional predicted risk and protective PCP gene variants found in 22q11.2DS patients may alter PCP signaling activity. Building upon our preliminary studies that PCP regulates cell polarity and polarized cell behavior to promote SHF morphogenesis and SHF cell deployment to the OFT, we will first determine how Tbx1 mutants may phenocopy the morphogenetic defects in PCP mutants, and whether constitutively activating PCP in the SHF may rescue SHF morphogenesis and OFT defects in Tbx1 mutants. Secondly, we will use CRISPR/Cas9 to create mouse models for three human PCP variants that we already discovered from 22q11.2DS patients and functionally validated in Xenopus, and test how variants dampening or enhancing PCP activity may impact OFT malformations in Tbx1 mutant mice. Finally, using a quantitative assay we established in Xenopus, we will systematically analyze a large number of rare predicted PCP gene variants associated specifically with 22q11.2DS individual either with or without OFT defects, and perform cellular and molecular studies to determine mechanistically how these variant alter PCP signaling. These studies will elucidate whether perturbation of PCP signaling is a key pathogenic mechanism of OFT malformation in 22q11.2DS, and how PCP gene variants may impact human biology and human health.